The present invention concerns a method for calibrating a tool used on a contact type measuring machine, said machine including a mobile measuring head on which said tool is fixedly mounted, the tool including a support rod carrying a contact element or tip at a first end, and connection means assuring the link with said measuring head at the second end.
The invention relates more precisely to the implementation of such a method for calibrating a touch probe type measuring tool.
The use of a reference object for calibrating machine-tools including a measuring head carrying a touch probe has been known for a long time. Indeed, the use of such a reference object whose dimensions are known very precisely enables said measuring head to be calibrated with respect to three dimensional spatial coordinate measurements.
However, the type of calibration described above has certain uncertainties capable of adversely affecting the precision of subsequent measurements. Indeed, calibration performed in the prior art enables the spatial origin of measurements to be fixed but does not take account of certain uncertainties, such as uncertainties related to deformations of the touch probe which are added to the inherent measurement uncertainties. The measurement uncertainty xe2x80x9cbudgetxe2x80x9d for such a machine is thus increased since it is calculated to take account of the uncertainty associated with the final result, when said uncertainty is maximum. Consequently, the slightest decrease in one of the uncertainties occurring in a measuring method causes a corresponding decrease in the uncertainty budget.
Uncertainties affecting deformations of the touch probe are very slight, but because of the rapid development in technology and the increase in precision available in manufacturing and measuring methods, there is a permanent demand to limit all types of uncertainty as far as possible.
The measuring tool, or touch probe, is generally formed of two or three parts, which are connecting means, a support rod and a contact element. The uncertainties described above originate from the use of a contact element whose geometrical features are not ideal and from the various methods used for assembling the touch probe components.
The method currently used for assembling touch probe components is bonding, using an adhesive material. However, bonding has a drawback in that the distribution of the adhesive material on the two surfaces to be bonded prior to assembly cannot be controlled in a reliable manner. Thus, the orientation of the various components of the touch probe in relation to each other varies from one probe to another, as does the quality of adhesion. Consequently, the dynamic behaviour of two, theoretically identical, probes will not be the same, more particularly as regards bending.
The main object of the present invention is thus to overcome the drawbacks of the aforementioned prior art by providing a method enabling deformations of touch probe during the measurement cycle to be taken into account.
The invention therefore concerns a method for determining the amplitude of the deformations that a measuring tool of the type indicated hereinbefore undergoes in common use, such method including the steps consisting in:
a) fixing said touch probe onto a support by said connecting means,
b) applying a force to a point of application located on a first part of said touch probe,
c) measuring the deflection magnitude of said first part of said touch probe using a first measuring device,
d) repeating steps a) to c) a plurality of times each time changing the points of application of the force and corresponding measurement, so as to obtain a radial distribution of the value of said bending which said touch probe undergoes,
e) providing a report representative of the set of measurements obtained by implementing steps a) to d).
By xe2x80x9ccommon usexe2x80x9d is to be understood the average usual conditions undergone by the touch probe during a surface scanning or measuring cycle.
In a preferred embodiment, the force is applied on a first part of the contact element, whereas said measuring device is arranged on a second part of said contact element, located facing said first part. Furthermore, in the event that said measuring tool is a touch probe whose contact element is a ball, step b) above consists in applying pressure to a first point located substantially on the equator of said ball, whereas the corresponding magnitude of deflection of the ball is measured, using a comparator, in proximity to a second point of said ball diametrically opposite said first point. Consequently, step c) above then consists in calculating the bending which the touch probe undergoes in conditions close to real conditions of use.
Moreover, repeating steps b) and c) above a plurality of times enables the bending of said touch probe in all directions to be determined and thus radial distribution of the bending to be obtained, enabling the dynamic behaviour of said probe to be known entirely.
A second comparator may also be used during the measuring steps enabling the magnitude of deflections of said probe""s connecting means to be determined with respect to the support and with respect to the contact element.
Likewise, a third comparator can be used enabling the magnitude of deflections of the end of said support rod located in proximity to the contact element, in particular with respect to the contact element itself to be determined. In this way, one can obtain precise, complete and detailed knowledge as to the dynamic behaviour of the touch probe. This advantageously enables one to determine more precisely than at the current time what uncertainties are involved as regards the measuring head and thus to greatly reduce the uncertainty budget of said measuring head. It is of course understood that in such case, the more one wishes to carry out measurements on a probe, the more advantageous it is to use an automatized method.
Consequently, the manufacturer of such measuring probes can define the dynamic behaviour of each probe which he wishes to market and provide a report representative of the results obtained in the form of a certificate, guaranteeing the level of uncertainties associated with said probe to the client. The user can thus combine this information with that subsequently obtained when the reference object is measured, to define the overall level of uncertainty of his machine-tool exactly.